“Green Flash” is a rare optical phenomenon which may occur during sunset or sunrise. It usually occurs as predominantly green coloration lasting a very short duration of around 1 to 2 seconds, at the extreme upper edge of the Sun (Figure 1).

The formation mechanism of the “Green Flash” is quite complex. Within an air layer of same temperature, light travels through it in a straight line. However, when the air temperature changes with altitude, light is refracted and appears to travel along a curve. Since different colors of light have different refractive indices, blue or green light is refracted more than the light of other colors. As blue light is likely to be scattered in the sky, green light may happen to be easily observed for a very brief moment, resulting in a green spot at the extreme upper edge of the Sun during sunset or sunrise.

One type of “Green Flash” is called the “Inferior Mirage Flash”, which can occur when the air near the surface is significantly warmer than the air above. As the sun approaches the horizon during sunset, the condition of warm air lying beneath cold air favors the formation of mirage, i.e. an inferior mirage (virtual image of the Sun) appearing below the Sun. As the Sun continues to approach the horizon, the Sun and its inferior mirage will overlap, forming a shape resembling the Greek alphabet "Omega"(Figure 2). When the upper edge of the Sun reaches very close to the horizon, the previously inconspicuous green light becomes more visible due to overlap with its inferior mirage, allowing observers to briefly witness the “Green Flash” with the naked eye.

Next time when you enjoy watching a sunrise or sunset, you might pay attention if you can witness the rare "Green Flash."